1. Field of the Invention
The invention relates to a method of imaging a pattern arranged in the object plane of an optical imaging system into the image plane of the imaging system, and an imaging system for carrying out the method. The preferred area of application of the invention is projection objectives for microlithography.
2. Description of the Prior Art
Catadioptric or catoptric projection objectives are used in projection exposure installations for the production of semiconductor components and other finely structured components. They have at least one concave mirror and are used to image patterns of photomasks or graduated plates, which in the following text will generally be designated masks or reticles, onto an object arranged in the image plane of the projection objective and coated with a light-sensitive layer, at the highest resolution and on a reducing scale.
Since the resolving power of optical imaging systems is proportional to the wavelength λ of the light used and inversely proportional to the image-side numerical aperture (NA) of the optical imaging system, in order to produce finer and finer structures, the aim is firstly to enlarge the image-side numerical aperture of the projection objectives and secondly to use shorter and shorter wavelengths, preferably ultraviolet light with wavelengths of less than about 260 nm.
In addition to the resolving power, the depth of focus (DOF) that can be achieved during the imaging plays an important role for imaging that is faithful to the original. The depth of focus is likewise proportional to the wavelength used but inversely proportional to the square of the numerical aperture. Therefore, any increase in the numerical aperture without suitable measures for providing an adequate depth of focus is expedient only to a limited extent.
In order to improve the resolution and the depth of focus of microlithographic projection objectives, it is known to use pupil filters. The use of pupil filters is sometimes also designated optical filtering or apodisation. Here, pupil filter designates a spatial filter which is arranged in the region of a pupil surface of a projection system. This pupil surface, which is generally flat, is a Fourier-transformed surface in relation to the object plane and to the image plane. This means, for example, that a specific angle of incidence of light in the image plane of the projection objective corresponds to a specific radial coordinate in the pupil plane. With the aid of locally resolving filtering in the region of the pupil surface, it is therefore possible to exert an influence on the angular spectrum of the rays contributing to the imaging.
U.S. Pat. No. 5,222,112 shows a purely reflective catoptric projection objective for soft X radiation in which, in the region of a pupil, there is arranged a convex mirror whose mirror surface has a reflectance which decreases from the center toward the edge. As a result, an amplitude filter having a fixedly predefined, rotationally symmetrical filter function is created. In addition to amplitude filters which, for example, mask out specific different diffraction orders in the region of the pupil surface, phase filters are also known which, according to a predefined local filter function in the pupil surface, effect phase shifting of the light passing through in specific zones of the pupil surface. An explanation of the functioning of pupil filters and also examples of projection objectives having such filters are shown, for example, in EP 0 485 062 B1 or U.S. Pat. No. 5,144,362 and in the texts cited there.
In order to achieve the optimum benefit of pupil filtering, it is expedient to adapt the mode of action of a pupil filter, determined by the construction of the pupil filter, to the type of reticle structures to be imaged. Accordingly, pupil filters are optimized for specific reticle structures (for example contact holes, grid structures with one or more periodicity directions). Since reticles with an extremely wide range of structures are intended to be imaged by a projection objective, it is desirable to be able to use pupil filters with different effects as desired. For this purpose, U.S. Pat. No. 5,610,684 discloses a projection objective which has a changer for interchanging pupil filters in the pupil plane of the projection objective. The changer comprises displacement devices for displacing lenses close to the pupil, in order to provide sufficient space for the interchanging process. In EP 0 638 847 B1 (corresponding to U.S. Pat. No. 5,448,336), a projection objective having a pupil filter interchanging device is shown, the operation of which requires no movement of lenses close to the pupil. The technical implementation of interchanging devices of this type is very complicated in high-performance projection objectives, since close tolerances for material, fit and thickness of the optical components used and high requirements on the positioning accuracy and, possibly, gas tightness are imposed.